This invention relates to improved particulate inks for use in ink jet printing processes. More particularly, this invention relates to ink jet inks that can be used in various printing processes such as thermal ink jet and piezoelectric or acoustic ink jet processes to provide archival print quality comparable to that obtained in xerographic toner development systems.
Ink jet printing processes and apparatus for such processes are well known in the art. Major types of ink jet processes are thermal ink jet and acoustic or piezoelectric ink jet processes.
In thermal ink jet printing processes, the printer typically employs a resistor element in a chamber provided with an opening for ink to enter from a plenum. The plenum is connected to a reservoir for storing the ink. A plurality of such resistor elements is generally arranged in a particular pattern, called a primitive, in a printhead. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled toward a print medium, such as paper. The entire assembly of printhead and reservoirs comprises an ink jet pen. In operation, each resistor element is connected via a conductive trace to a microprocessor, where current-carrying signals cause one or more selected elements to heat up. The heating creates a bubble of ink vapor in the chamber, which, in turn expels a droplet of ink through the nozzle toward the print medium. In this way, firing of a plurality of such resistor elements in a particular order in a given primitive forms alpha numeric characters, performs area fill, and provides other print capabilities on the medium. The thermal ink jet printing process is described in more detail, for example, in U.S. Pat. Nos. 5,169,437 to You and 5,207,824 to Moffatt et al., the entire disclosures of which are incorporated herein by reference.
In an acoustic or piezoelectric ink jet system, ink droplets are propelled to the recording medium by means of a piezoelectric oscillator. In such a system, a recording signal is applied to a recording head containing the piezoelectric oscillator, causing droplets of the ink to be generated and subsequently expelled through the printhead in response to the recording signal to generate an image on the recording medium. In this printing system, a recording signal is converted into a pulse by a signal processing means such as a pulse converter and then applied to the piezoelectric oscillator. A change in pressure on the ink within an ink chamber in the printhead caused by the recording signal results in droplets of ink being ejected through an orifice to a recording medium. Such an ink jet system is described in more detail, for example, in U.S. Pat. No. 4,627,875 to Kobayashi et al., the entire disclosure of which is incorporated herein by reference.
A related printing method is the impulse, or drop-on-demand, ink jet printing process. Such impulse ink jet printing processes generally use a hot melt ink jet ink. For example, in the impulse printing process, the hot melt ink is heated into a fluid phase and is caused to form a convex meniscus at the printhead nozzle tip by hydrostatic pressure. This pressure causes the end of the ink bubble to intrude into an electrostatic field. The ink is then electrostatically drawn into a single file stream of drops that traverse the span between the tip of the nozzle and the carrier (paper, etc.). Such impulse printing processes, and hot melt inks for use therein, are described, for example, in U.S. Pat. No. 4,659,383 to Lin et al., the entire disclosure of which is incorporated herein by reference. A disadvantage of the impulse printing processes, however, is that the resultant print image has a waxy texture and appearance, sometimes referred to as crayoning, and which may smear when abraded due to the presence of wax in the hot melt ink jet ink. The resultant print image thus generally has a lower archival quality.
In these and other ink jet recording processes, it is necessary that the ink being used meet various stringent performance characteristics. Such performance characteristics are generally more stringent than those for other liquid ink applications, such as for writing instruments (e.g., a fountain pen, felt pen, etc.). In particular, some or all of the following conditions are generally required for inks utilized in ink jet printing processes:
(1) the ink should possess liquid properties such as viscosity, surface tension and electric conductivity optimized for the discharging conditions of the printing apparatus, such as the thermal ink jet heater temperature rise, the driving frequency of a piezoelectric electric oscillator or a thermal ink jet head, the geometry and materials of printhead orifices, the diameter of orifices, etc.
(2) the ink should be capable of being stored in the device for a long period of time between duty cycles without causing clogging of printhead orifices during use. In the most stressful case the device should be able to sit uncapped for hours or days and still be able to recover all jets when fired.
(3) the recording liquid should be quickly fixable onto recording media, such as paper, film, etc., such that the outlines of the resulting ink dots are smooth and there is minimal smearing of the printed image.
(4) the resultant ink image should be of high quality, such as having a clear color tone and high optical density. The ink image should also have a large color gamut ideally, equal to or better than, that obtained with laser xerographic printers.
(5) the resultant ink image should exhibit excellent waterfastness (water resistance) and lightfastness (light resistance).
(6) the ink should not chemically attack, corrode or erode surrounding materials such as the ink storage container, printhead components, orifices, etc.
(7) the ink should not have an unpleasant odor and should not be toxic or flammable.
(8) the ink should exhibit low foaming and good shelf life stability characteristics for properties such as particle growth, viscosity creep, pH stability, etc.
Various inks for ink jet printing processes are known in the art. For example, various ink jet inks are disclosed in U.S. Pat. Nos. 4,737,190 to Shimada et al. and 5,156,675 to Breton et al. Generally, the ink jet inks of the prior art are aqueous inks, comprising a major amount of water, a humectant and/or a co-solvent, and colorant. By selecting specific components such as humectants, colorant, or other components, it is possible to adjust the print characteristics of the resultant ink.
Another widely-used imaging method, which produces high (archival) quality images and prints, is known as xerography or electrostatographic imaging. Xerography generally includes processes such as electrophotographic and ionographic imaging. Such processes use dry or liquid toner and/or developer compositions to develop images.
Generally, the process of electrostatographic imaging includes the step of forming a charge on an imaging member in the form of an image, such as an image of an original document being reproduced, or a computer generated image written by, for example, a raster output scanner. This records an electrostatic latent image on the imaging member corresponding to the original document or computer-generated image. The recorded latent image is then developed by bringing oppositely charged toner particles into contact with it. This forms a toner powder image on the imaging member that is subsequently transferred to a substrate, such as paper. Finally, the toner powder image is permanently affixed to the substrate in image configuration, for example by heating and/or pressing the toner powder image. Such xerographic imaging processes are described in, for example, U.S. Pat. Nos. 4,762,674, 5,019,477 and 5,254,427.
Various xerographic imaging methods utilizing a black or colored toner composition produce images and prints having very high quality. Such images and prints generally have high fix, i.e., smear resistance, as well as excellent waterfastness and lightfastness, making such imaging methods preferred where archival quality images and prints are necessary. Such high archival quality has generally not been obtainable using ink jet printing processes. However, the ink jet printing processes possess an advantage over the xerographic imaging processes, in that the process and apparatus used in an ink jet printing process are generally cheaper and less cumbersome than in xerographic development processes. For example, both the material and operating costs of an ink jet printing process are generally cheaper than for a xerographic development process.
Accordingly, ink jet printing has become one of the fastest growing segments in the low volume printer market. However, ink jet printing systems, and particularly thermal ink jet printing systems, suffer from several print quality shortfalls as compared to xerographic development systems employing polymer resin based toners. These shortfalls include the following:
(1) Lack of 100% waterfastness in black and color inks. Although advances have recently been made to improve waterfastness, many of the ink jet ink compositions presently on the market do not achieve 100% waterfastness, and therefore are subject to smearing, line blooming, or other problems in high humidity atmospheres and the running of colors when prints come into contact with water.
(2) Lack of edge sharpness. It is important that the images exhibit high edge acuity, being sharp rather than ragged. Although some printers have addressed this problem by incorporating multipass printing and elaborate pixel management algorithms and/or heating of the imaging substrate, some edge raggedness still exists with the dye based color inks used in these printing systems.
(3) Inter-color bleed. Water-based ink jet printing processes suffer from a phenomenon known as inter-color bleed. This phenomenon is the bleeding of color from one freshly printed area into another at a boundary line before the water-based inks dry. This is most noticeable to the human eye when black text is printed on a yellow highlight background. Because the inter-color bleed occurs in much less than one second after printing, it cannot be eliminated by drying the imaging substrate after printing has occurred. Some printing systems have addressed the problem by controlling the printing process so that there is no "wet-on-wet" printing. However, this tends to slow down the printing process.
(4) Lack of saturation in colors on plain (so called office) paper. In ink jet printing processes, the highest quality prints have generally been obtained by printing on specially-designed coated papers. However, when standard uncoated papers are used, without heating of the paper before or during printing, the color saturation is lower, resulting in less acceptable prints. Thus while specially-designed coated papers may produce higher quality prints, they also increase the materials cost for ink jet printing processes.
(5) Show through. One reason for the reduced print quality of water based ink jet printing processes is that the colorant penetrates further into the surface of absorbent imaging substrates, such as paper. A direct result of the increased penetration into the paper surface is the problem known as "show through," i.e., the increased visibility, relative to xerographic prints, of the image from the back side of the paper. In addition to being objectionable in itself to some users, the show through problem also severely limits the possibility for duplex printing using ink jet inks on such papers.
(6) Paper latitude. Ink jet printing processes using water-based ink generally exhibit a smaller paper latitude as compared to xerographic imaging processes. For example, in printing processes that use only ambient temperature drying, the paper latitude is the worst. In such cases, paper variation leads to significant variation in print quality.
Although numerous water-based ink jet inks are presently available, they generally do not meet all of the above-described functional requirements to work well in ink jet printers, while also providing excellent print quality on plain paper. In particular, the inks generally used in ink jet printing processes, while producing acceptable print quality, do not produce the high print quality that is achieved by using toner compositions, such as in electrostatographic imaging processes.
Thus the need continues to exist in the ink jet industry for improved ink jet inks that satisfy the above-described functional requirements while at the same time providing high print quality prints with archival properties on a wide variety of recording media, including plain paper, transparencies and cloth. Especially useful are ink jet ink compositions that can provide archival quality prints equivalent to laser xerography without suffering from the above-described problems generally associated with ink jet inks.